3D animated glass object in video -> how to add reflection
Tough one, in the reflection you would see the person holding the tank, but from a different perspective (arms. chest and head and the sky behind).

So to get an accurate reflection, you would need to generate a 3d character that matches the person holding the tank, or shoot some video from a low angle that contains the image that would be reflected and map that onto the surface of the water.
The cheapest (and inaccurate) way to do this would be to create a surface that reflects a plane with the current image mapped to it. That plane would need to be invisible to the camera but visible to glossy rays.